Metallic and composite structures such as aircraft cockpits or boat hulls are usually covered on the inside with protection having thermal and acoustic barrier characteristics in order to insulate the inside of the cockpit or of the hull from the outside environment.
To this end, proposals have been made for protection systems, which are usually in the form of a mat consisting essentially of one or several layers of glass fibres enclosed in a sheath. This sheath can be produced from any type of material. Preferably, it is a film made of an organic material, such as polyester, polyimide etc. having at least hydrophobic characteristics and acting in certain cases as a water-tight barrier. By way of example, one may mention as a material used to produce the sheath aluminised or unaluminised mylar®, tedlar®, which is produced in a film of PVF, kapton®, which is a film produced from polyimide (registered trademarks of Dupont), or other coverings, such as polyester or polyamide films, such as textril®, which is a polyester film reinforced with polyethylene fibres made by the Jehier company. These films forming the sheath must be produced from materials that allow the customary textile treatments: stitching, bonding, welding etc. and have mechanical characteristics such as resistance to tearing etc.
On the other hand, reflecting a concern to minimise weight, the density of these various materials should be as low as possible while at the same time allowing superior mechanical characteristics to be achieved. The weight per unit area of this type of protection is preferably less than 100 g/m2.
It is absolutely essential to protect as far as possible the passengers inside an aircraft from the risk of ignition of the fuel generally coming from external engines. Indeed, when it ignites, this fuel, such as kerosene, reaches temperatures well above 1000° C. Due to this fact, it is advisable to protect the elements of metallic and composite structures forming aircraft cockpits, boat hulls, outside structures of trains etc.
To this end, the authorities and, in particular, the FAA (Federal Aviation Administration) have established relatively strict fire protection standards. However, the standards to which aircraft manufacturers have to conform are continually evolving and are becoming ever more stringent, reflecting a concern of increased safety of travellers.
The fireproofing characteristics of the protection systems described above, which belong to the prior art, are nowadays found to be inadequate. The transport department of the FAA has therefore attempted to publish test criteria appropriate to the new requirements. In particular, the characteristics of resistance to the “burn-through test” and the “inflammability test” were redefined in September 2000 in standard 14 CFR, part 25 et al.
In particular, the “burn-through” test consists in subjecting the mat of fibres and its sheath to the flame of a burner. The said burner supplies an impinging flame at a temperature of around 1150° C. The sample is thus subjected to a heat flux of 149 kW/m2. The product concerned will satisfy the requirements of the FAA if it succeeds in resisting penetration by the said flame for 4 minutes and if the heat flux produced by the sample is less than 23 kW/m2, measured at a distance of 30.5 cm (12 inches) from the impingement surface.
The inflammability test (ASTM-E 648), which consists in subjecting a sample measuring 1000 mm in length and 250 mm in width to a radiant panel sloping at 30° in front of the sample and in the presence of a pilot flame. The radiant panel produces a heat flux of 18 kW/m2 and ignition is effected by means of a pilot flame. The criteria for passing the test are the absence of flames within a radius of 51 mm around the point of application and the absence of post-combustion after extinction of the pilot flame for a specific test period.
Aircraft manufacturers have likewise defined certain mechanical specifications, such as flexibility and tensile strength, and their variation, obtained as a result of standardised conditioning or ageing of the samples.
On the other hand, the prior art, in particular the document EP-A-0370337, has disclosed the use of an impregnated mica paper, possibly bonded to a support based on woven or nonwoven glass fibres, aramid fibres, carbon fibres or some other type, such as a fireproofing covering with a low rate of heat release for construction elements in applications subject to relatively stringent standards in this matter, such as the aeronautical industry, the automotive industry, interior decoration etc.
Although this type of use for mica paper initially satisfied the standards in force in the 1980s (ATS 10 333-001, directive FAR 25—OSU chamber), this type of covering does not satisfy the new safety standards, such as those defined above.
It is likewise known, in particular from the documents EP-A-0949367, FR-A-3 884 337, EP-A-0406467 and U.S. Pat. No. 4,514,466, that mica and, in particular, mica paper is a good electrical insulator and has good heat resistance. Nevertheless, these documents do not mention the use of mica for a fire protective barrier applied along a metallic or composite structure such as an aircraft cockpit, a boat hull or the outside structure of a train etc.
It will furthermore be noted that, in all the prior-art applications, the conventional mica paper has a weight per unit area close to 100 g/m2.